Platinum-iridium coated electrodes of the type described in U.S. Pat. No. 3,177,131 have found widespread use in electrolytic production of chlorine and other chlorine-containing products. These anodes possess three of the four qualities which are particularly desirable in commercial electrodes: long life, high efficiency and high selectivity. However, the fourth desirable quality, low initial cost, is not obtained, as the initial cost of these anodes is quite high compared to the relatively inexpensive materials which were used for anodes in electrochemical applications early in this century. For instance, graphite anodes were used to produce chlorine and lead anodes were used for electrowinning. Even though these materials had rather limited life and only moderate efficiency, they were extensively used because they were inexpensive and readily available.
Even though modern precious metal catalytic electrodes have a high initial cost, they can be considered economical in view of their high efficiency and long life. When used in a typical diaphragm cell for producing chlorine, life is measured in terms of years, with from about 3 to 5 years being considered typical, and more than 10 years exceptional. Shorter lives are considered undesirable, since in addition to labor, the cost of the anodes replacement requires an extended shut-down of the cell and a consequent loss of production.
The efficiency of precious metal electrodes is generally guite good. Efficiency is typically thought of in terms of the amount of electrical energy required to produce a given quantity of the desired product. This depends primarily on two factors, over-voltage and selectivity. Activation over-voltage is the difference between electrical potential required to obtain the desired production at the operating current and the standard half-cell potential for the desired reaction. In chlorine production, under typical conditions, the activation over-voltage of precious metal-coated electrodes is typically less than about 100 millivolts over the life of the electrode. This represents about a 4% loss in a typical process running at 3 to 4 volts. When the activation over-voltage exceeds about 200 millivolts, the power savings obtainable with new electrodes could be as high as about 8%, which might normally justify replacement. For comparison, the over-voltage obtained with graphite electrodes typically ranges from about 300 to about 500 millivolts.
Selectivity of an electrode is the ability of the electrode to preferentially catalyze the reaction which produces the desired products. In chlorine production, precious metal-coated electrodes typically produce about 1 to 3 v/o (volume percent) oxygen, which is undesirable, and lesser amounts of undesirable chlorine-oxygen compounds. The literature reports that, under similar conditions, graphite electrodes typically produce about 4% oxygen along with chlorinated organics and carbon dioxide. Since the raw material for chlorine is common salt, the major production expenses are the capital cost of the plant and the cost of the electrical energy for the electrolysis. Thus, even though more modern precious metal-coated electrodes initially cost substantially more than obsolescent graphite electrodes, they are ultimately more economical, as they are more efficient.
The platinum-iridium anode coatings of commerce are normally specified as containing 70% Pt:30% Ir by weight of the metal, even though the metals are thought to form at least a thin surface film of oxide which catalyzes the electrolysis reaction with the iridium being present throughout as the oxide. These anodes are prepared by making a solution of salts or resinates of the two metals in an appropriate solvent, such as alcohol, for salts or a mixture of essential oils for resinates, applying multiple thin coats, and firing in air or other oxidizing atmosphere between 350.degree. and 550.degree. C. Unfortunately, the cost of iridium is comparable to that of platinum, and both have escalated rapidly in recent years. In chlorine production, platinum-iridium-coated anodes generally evolve less than about 1% oxygen, which is considered very good.
However, some of the platinum group metals, while still precious, are considerably less precious than platinum, and further, their prices have not risen as rapidly as that of platinum; so in the 60's, ruthenium oxide-coated anodes became commonly used, while platinum-iridium was used on a smaller scale. The commercial ruthenium oxide coatings are thought to be prepared as described in U.S. Pat. No. 3,632,498, by dissolving RuCl.sub.3. 1-3H.sub.2 O and TiCl.sub.3 or TaCl.sub.5 in alcohol, applying thin coats of the solution to titanium, and firing each coat in air in the temperature range of 350.degree.-600.degree. C. However, in chlorine production, the ruthenium oxide-coated electrodes produce slightly more oxygen than Pt-Ir electrodes. This represents a considerable waste of electrical energy and may require further purification of the chlorine. Ruthenium oxide-coated electrodes, such as are known to the prior art, are not generally considered suitable for use in many acidic media, such as are encountered in electrowinning.
Precious metal-coated titanium anodes are usually dimensionally stable and have very long life in brine electrolyses, as compared to graphite, but there is a need to obtain alternative coatings incorporating smaller amounts of the more expensive precious metals while achieving comparable or even longer life at lower cost, or at least without unduly increasing the cost of the coating. Further, high selectivity must be maintained.
In the case of platinum-iridium, customers may typically specify coating loading in the range of 5-20 grams per square meter in expectation of a life of more than 7 years in a chlorate cell. Multiple coating applications may be needed to obtain this loading range, so cost of application can be a significant part of the overall cost. Longer life could be obtained by increasing the loading further, but as a practical matter, this is not usually done because of precious metal and labor costs.
Prior to this invention, it was suggested that powdered lead ruthenate could be used as an electrocatalyst in chlorine production in U.S. Pat. No. 3,691,052, which describes a technique of forming the catalytic coating by firing a mixture of a low melting glass and lead ruthenate at a temperature above the melting point of the glass. However, these coatings have not been found to be sufficiently durable to gain wide commercial acceptance. If higher melting glasses are used, the durability can be increased somewhat, but the extreme reactivity of titanium substrates presents difficulties in fabrication. As is well known, titanium is a powerful reducing agent and tends to oxidize very rapidly at elevated temperatures.